1. Field of Invention
The invention relates to improved electrolytic cells for the separation of alkali metals from electrodissociatable compounds in the molten state and more particularly to molten metal flow detection in such cells.
2. Prior Art
The metals most frequently made by electrolysis of electrodissociatable compounds in the molten state are the alkali metals, particularly sodium and lithium.
The type of electrolytic cell most widely used for the production of sodium metal is the Downs cell, which is described in U.S. Pat. No. 1,501,756 to J. C. Downs. The Downs-type electrolytic cell basically is comprised of a refractory-lined steel shell for holding the molten salt electrolyte, a submerged cylindrical graphite anode surrounded by a cylindrical steel cathode and a perforated steel diaphragm positioned in the annular space between the electrodes to separate the anode and cathode products. To collect product halogen gas from the anode, the cell is provided with collector means such as an inverted cone which fits over the anode below the surface of the molten bath. Halogen gas (usually chlorine) passes upwardly through the cone and, via appropriate manifold components, from the cell. Similarly, the cathode is also provided with collector means such as an inverted inclined trough which fits over the cathode below the surface of the molten bath. Molten alkali metal rises from the cathode toward the surface of the molten bath, is collected along the inclined surface of the trough and is passed to a vertical riser/cooler in which the molten metal is partially cooled before it is passed to a product receiver.
Despite the current technical and economic superiority of the Downs cell for making alkali metals, particularly sodium and lithium, the cell nevertheless has several disadvantages which are becoming even more highly significant as additional emphasis is placed on energy conservation and the quality of working environment for operating personnel.
A most promising route by which the disadvantages of the prior art can be overcome is to employ an electrolytic process in which a solid electrolyte material, which, under the influence of an electrical potential, is permeable to the flow of selected cations, but impermeable to the flow of other species, i.e., fluids, anions and other cations, to separate the anode and cathode compartments of the cell. A basic method for carrying out the electrowinning of alkali metals in this manner is disclosed in U.S. Pat. Nos. 3,404,036 and 3,488,271 to Kummer et al. in which a flat plate of sodium beta alumina is used as the solid electrolyte material. A similar method is disclosed in U.S. Pat. No. 3,607,684 to Kuhn in which sheets of beta alumina are used as a diaphragm to separate the anode and cathode compartments of the electrolytic cell.
Though the cells of the prior art, which have employed solid electrolyte material as a separator between the cathode and anode, are effective in carrying out the electrolytic separation of metals from molten salts thereof, such cells have remained largely undeveloped and lack the configuration necessary to obtain efficient continuous operation on a commercial basis. In particular, the cells of the prior art have not been of such design as to provide for safe continuous cell operation in the event of breakage of the fragile solid electrolyte material, nor do such prior art cells permit efficient use of electrical energy and factory floor space by providing an acceptable ratio of solid electrolyte surface area to cell volume.